1. Field of the Invention:
This invention relates to triggering devices mounted in motor vehicles, and more particularly, to such devices which include malfunction alarms.
2. Description of the Prior Art:
There have been proposed passenger cars equipped with passenger protective means in the form of, for example, gas-inflatable bags for protecting passengers in the event of a car crash. Such passenger protective means generally include a gas producing device incorporating a current-activated explosive. The gas producing device is activated by the shock of a car crash to produce the gas, so that the bag is inflated in front of the passenger to prevent the passenger from striking a hard object. Although a detonator known as a squeep, contained in the triggering device, is designated to set off explosives once current exceeding a predetermined value passes through it, it may as well never be installed should the wires connected to it break before a car crash. Accordingly a malfunction detecting circuit has been proposed to detect and warn of any malfunctions in its operation.
FIG. 1 is a schematic circuit configuration illustrating an example of a vehicle-mounted triggering device equipped with the conventional trouble detecting circuit disclosed according to the Japanese Patent Publication No. 23263/83. In vehicle-mounted triggering device 10 shown in FIG. 1, both ends of a detonator 12 within a gas producing device 14, supplied with a bag 16, are connected to a main power supply 18 through detecting means such as impact sensors 20 and 22. Each of sensors 20 and 22 includes a resistor R so that a very small current continuously flows through detonator 12. This current is monitored by comparators 24 and 26 in a trouble detecting circuit 28. The very small current is less than that necessary to ignite detonator 12.
Impact sensors 20 and 22 also include switches S which are closed by the shock of a crash. Switches S are connected in parallel with resistor R. A detonator circuit 30 includes impact sensors 20 and 22, detonator 12 and an auxiliary power supply 32. Auxiliary power supply 32 includes a capacitor C connected in series with the parallel combination of a charge resistor Rc and a discharge diode Dd. The terminals of auxiliary power supply 32 are connected to main power supply 18 through diodes D1 and D2 as unidirectional energizing elements which prevent auxiliary power supply 32 from discharging when the voltage of supply 18 drops as a result of an accidental short across the starter or head lamp wiring, for example. Also, an ignition switch SO is connected between the positive terminal of main power supply 18 and diode D1.
In trouble detecting circuit 28, a divider resistance circuit 34 includes a series circuit having resistors r1 and r2, each having roughly the same resistance as that of resistor R, and a resistor r0 having the same resistance as that of detonator 12. The series circuit is connected across main power supply 18. The voltage between resistors r1 and r0 is applied as a reference input to the non-inverting input terminal of comparator 24, whereas the voltage between resistors r0 and r2 is applied as a reference input to the inverting input terminal of comparator 26. The terminal voltage of detonator 12 is applied to both comparators 24 and 26 as a comparison input. An alarm lamp 36, which lights when trouble is detected, is connected between main power supply 18 and the output terminal of an AND gate circuit 38 receiving the outputs of both comparators 24 and 26.
Accordingly, if the terminal voltage of detonator 12 changes because of an accidental short or a disconnection originated from, for instance, impact sensor 20 or 22 or detonator 12 itself, the output from comparator 24 will become low when the terminal voltage of detonator 12 rises above a reference and the output from comparator 26 will become low when the terminal voltage of detonator 12 drops below a reference, causing AND gate circuit 38 to assume a low output and alarm lamp 36 to light. Thus trouble can be detected.
Suppose, however, a large load is suddenly applied to main power supply 18 such as occurs when the engine is started or the head lamps are lit. This causes the terminal voltage of main power supply 18 to drop instantly, whereas the terminal voltages of auxiliary power supply 32 and detonator 12 drop slowly due to capacitor C as shown in FIGS. 2(A) and 2(B). For this reason, as shown in FIGS. 2(C) and 2(D), the terminal voltage of detonator 12 becomes well over the reference input of comparator 24 which changes as sharply as the terminal voltage of main power supply 18, causing alarm lamp 36 to light during the period shown by .DELTA.T in FIG. 2(B). In other words, the problem is that a false alarm is given. In addition, a similar problem occurs with respect to comparator 26 when the voltage of main power supply 18 is abruptly increased.